Gas filter correlation radiometry is an optical remote sensing method used to produce highly sensitive measurements of a target gas present in an atmospheric region. In applying this method, a Gas Filter Correlation Radiometer (GFCR) instrument views a scene through a cell that includes a sample of the target gas to thereby create a high-resolution spectral “notch” filter. A “notch” filter is one that prevents the passage of light at narrow spectral locations as opposed to the typical filter that allows light to pass at one or more narrow spectral locations. The (target) gas in the cell will absorb light at exactly the spectral locations of the target gas absorption features to thereby create a filter that is almost perfectly correlated with the target gas. The target gas is detected by comparing measurements of total light over a limited spectral bandpass with and without the “notch” filter, or by comparing measurements with various “notch” filters (i.e., different gas concentrations in the gas cell of each “notch” filter). The former typically splits the beam creating multi-beams to make measurements, while the latter methods that compare signals by varying the “notch” filter typically use one beam and modulate the cell condition. Both measurement systems have a gas cell that includes a gas that is chemically identical to the target gas that is to be detected/measured.
Unfortunately, some target gases do not lend themselves to accurate detection and/or measurement using a GFCR instrument because the target gas's spectral absorption features are too weak or non-distinct to produce adequate absorption signatures in a gas cell environment and/or the target gas is chemically unstable in a gas cell environment. For example, detecting/measuring ozone with a GFCR instrument has been ineffective due to ozone's inherent instability that makes its containment in a GFCR system's gas cell problematic. In another example, the measurement of oxygen with a GFCR instrument is problematic because oxygen does not have sufficiently strong absorption features needed for a GFCR gas cell filter.